The present invention relates to a weighting circuit for an electrical torque signal developed by a hand tool or machine, particularly a drilling machine, having a first sensor device for comparing the amplitude of the torque signal with a predetermined level, wherein a trigger signal is produced by the circuit when the predetermined level is exceeded. The trigger signal then serves to switch off the machine. Such a circuit is known generally from DE-OS 24 42 260.
What is referred to below as an electrical torque signal is primarily an electrical signal which is produced by an error sensing device upon conversion of a mechanical value corresponding to torque developed in a machine, which electrical signal may possibly be further processed (for example, through an amplifier). Such electrical signal basically may be one like that produced, for example, by a foil or resistance strain gauge between the handle and the housing of a drilling machine. However, signals from an accelerometer or similar devices, which are produced by a sudden rotational movement of a drill housing caused, for example, by blocking of the tool, are also comprehended in this context. Through double integration, the acceleration signal of the accelerometer may be converted into a distance/time signal, and only then coupled to a weighting circuit.
In DE-OS 24 42 260, a signal is developed by a foil strain gauge arranged between the handle and the housing of a drilling machine so that the strain gauge serves as a torque measuring member. The developed signal influences the motor of the drilling machine and switches it off when a certain signal level is exceeded.
The circuit disclosed in DE-OS 24 42 260, however, does not sufficiently allow for complicated, practical conditions, so that random triggerings and failure of triggering often occur.
On the one hand, for example, blocking of a tool in approximately 10 ms can occur in a drilling machine during lateral drilling of a reinforcing iron, e.g., with a recess cutter, which time is less than the reaction time of the user, even for an anticipated occurrence (200-250 ms). Thus, in this case, the circuit and the active system influenced by the circuit (e.g., a clutch or coupling) must respond promptly enough so that an injury is avoided safely.
On the other hand, simple jamming of the tool can arise, which jamming occurs during every half revolution of the tool, but which should not lead every time to interruption of a drilling operation after a deliberate or conscious reaction of the user. After thorough measurements and observations were carried out, it was discovered that the initial occurrence of jamming is reflected by a relatively high but narrow torque peak. This arises because the handle is loosely held. In the event of jamming, the handle rebounds severely against the mass of the user's hand and arm. After the muscles are tensed, the transmission of force to the machine handle-hand-arm combination is cushioned and damped. Torque peaks which are weaker than the initial peak then occur. A practical trigger circuit therefore should also be able to allow for this torque characteristic. The rise time, (i.e., the time during which an edge of the tool is engaged with, e.g., a reinforcing iron during which time the increased torque acts), is approximately 120 ms during jamming.
Furthermore, so-called "twisting", i.e., a torque increase on the machine handle, can also occur with a relatively long rise time of 1000 ms. This arises, for example, through tangential drilling on iron, inhomogeneities in concrete, or poor guiding of the machine. The trigger circuit thus should allow a certain amount of twisting, but should nevertheless respond to a sudden blocking or severe overload.